1. Field of the Invention
The present invention generally relates to a method and system for vagus nerve stimulation for cardiac defibrillation, and more particularly, the present invention relates to a method and system for vagus nerve stimulation for cardiac defibrillation during ventricular fibrillation.
2. Description of Related Art
Preventative and protective treatment against cardiac arrhythmias including atrial fibrillation and ventricular fibrillation include vagus nerve stimulation (VNS). In one study, anesthetized dogs underwent coronary occlusion while undergoing medium and high intensity vagus nerve stimulation. The results were that hearts took longer to enter ventricular fibrillation (VF) during vagus nerve stimulation compared to controls. In another study vagus nerve stimulation (20 Hz trains) in anesthetized dogs lowered the threshold for intensity of shocks necessary for defibrillation.
Cardiac defibrillation with vagus nerve stimulation may include vagus nerve electrical stimulation providing protection against induced heart fibrillation/arrhythmia, and to lower the intensity of external electric shocks needed to defibrillate a fibrillating heart. However, defibrillator devices may require insertion of electrodes into the heart and ambulatory electrical defibrillator devices which require the insertion of electrodes into the heart are invasive, that is they invade the heart itself.
Existing implantable defibrillator devices that are placed via catheter to reach the ventricular lining in one or more locations have elements that cross one or more valves of the heart. Existing implantable defibrillators placed via access through a blood vessel, have an element of the device crossing the vessel wall. Thus, these devices have a risk of bleeding associated with the implant, and also a risk of causing valve damage.
Further, current use of vagus nerve stimulation is for activation of afferent nerve fibers (i.e., fibers intended to carry signals into brain). Afferent fibers are selected by the stimulus parameters because these fibers have a lower threshold for activation. Higher intensity stimuli required for activation of efferent vagal fibers (i.e., coming from brain to heart or other organs) will activate all vagal fibers (i.e., the high threshold efferent fibers cannot be selectively activated by adjusting stimulus parameters).
For illustrative purposes, referring to FIGS. 10A and 10B, a prior art implantable cardioverter-defibrillator (ICD) 352 (controller unit), 354 (wires), 360 (electrode) is shown in FIG. 10A, and a prior art pacemaker 362 (controller unit), 354 (wires), 360 (electrode) is shown in FIG. 10B. FIG. 10A shows the location and general size of the ICD 352 in the upper chest of a patient 358. Wires 354 with electrodes 360 on the ends are inserted into the heart 356 through a vein in the upper chest. FIG. 10B shows the location and general size of the pacemaker 362 in the upper chest of the patient 358. The wires 354 with electrodes 360 on the ends are inserted into the heart 356 through a vein in the upper chest of the patient 358.
Defibrillators are most commonly implanted through small incisions in the skin, near the collarbone. The patient is given a mild sedative, but is kept awake; a local anesthetic may be used. An electrophysiologist, a physician who specializes in heart rhythm problems, typically performs the procedure.
Using a fluoroscopy machine, which creates moving X-rays to visualize the chest's interior of a patient, an electrophysiologist guides the defibrillator leads through the incisions, into a large blood vessel, until they reach the heart. In the heart, the tip of each lead is attached to the heart muscle. Next, the leads are connected to the device's pulse generator, which is then placed in a pocket, just beneath the skin in the upper chest.
With the defibrillator in place, the electrophysiologist will perform a series of tests to ensure that device is working properly. An artificially fast heart rhythm might be programmed, e.g., fibrillation of the heart, to ensure that the leads are properly monitoring heart beats, and the pulse generator is sending the appropriate signals to stop the abnormal heart beat, e.g., defibrillation. Afterward, the device is programmed to meet the patient's needs. In this example, using existing ICD and defibrillation technology (e.g., implementing a shock to the heart), the device, the ICD, configuration includes electrically shocking the heart to causing fibrillation of heart for a period of time to configure the device for defibrillation. Thus, one disadvantage of this system for treatment is that the configuration technique of existing ICD devices is very stressful and dangerous for the patient by causing fibrillation during configuration.
When ventricular arrhythmias occur, the heart may not be able to pump blood well, and a patient can pass out within seconds and die within minutes if not treated. To prevent death, the arrhythmia must be treated immediately with an electric shock to the heart. This treatment is called defibrillation. An ICD has wires with electrodes on the ends that connect to a patient's heart chambers. The ICD can monitor a person's heart rhythm. If the device detects an irregular rhythm (e.g. ventricular tachycardia) in a person's ventricles, it will use low-energy electrical pulses to pace the heart in an attempt to restore a normal rhythm. If the low-energy pulses don't restore the person's normal heart rhythm, the ICD will switch to high-energy pulses for defibrillation. The device also will switch to high-energy pulses if the ventricles start to quiver (i.e., fibrillation) rather than contract strongly. The high-energy pulses last only a fraction of a second, but they can be painful, and therefore undesirable. In some instances, the pain from high-energy pulses can be severe, especially in cases of an administered defibrillation shock, such pain from repeated shocks can result in post traumatic stress disorder.
Doctors may also treat arrhythmias with another device called a pacemaker. An ICD is similar to a pacemaker, however, pacemakers give off only low-energy electrical pulses. They're often used to treat less dangerous heart rhythms, such as those that occur in the upper chambers of the heart. Some ICDs can act as both pacemakers and defibrillators.